Enhanced hvpmos

ABSTRACT

A p-channel LDMOS device with a controlled n-type buried layer (NBL) is disclosed. A Shallow Trench Isolation (STI) oxidation is defined, partially or totally covering the drift region length. The NBL layer, which can be defined with the p-well mask, connects to the n-well diffusion, thus providing an evacuation path for electrons generated by impact ionization. High immunity to the Kirk effect is also achieved, resulting in a significantly improved safe-operating-area (SOA). The addition of the NBL deep inside the drift region supports a space-charge depletion region which increases the RESURF effectiveness, thus improving BV. An optimum NBL implanted dose can be set to ensure fully compensated charge balance among n and p doping in the drift region (charge balance conditions). The p-well implanted dose can be further increased to maintain a charge balance, which leads to an Rdson reduction.

TECHNICAL FIELD

This subject matter is generally related to semiconductor devices, andmore particularly to p-channel lateral double diffusedmetal-oxide-semiconductor (LDMOS) for high voltage applications.

BACKGROUND

P-channel LDMOS is widely used as high side power devices due to reducedgate drive circuitry. When combined with n-channel LDMOS, p-channelLDMOS can be employed in level shifters in many applications such asmotor drivers or display panels. A good specific on-stateresistance/breakdown voltage (Rdson/BV) trade-off of n-channel LDMOS ispossible due to the Reduced SURface Field (RESURF) principal since thehandle wafer is grounded and drain forward biased. However, forp-channel LDMOS, the RESURF principle is inhibited because the drain andhandle wafer are commonly biased to the same potential.

Some conventional designs have been developed to overcome this issue. Inboth Bulk and thick film silicon-on-insulator (SOI) technology, thevertical depletion can also be assured with a source metallization orgate polysilicon which acts as a field plate. The presence of the n-typefloating layer associated with the field plate defines a double-RESURFeffect which leads to a competitive Rdson/BV tradeoff. In a thin filmSOI substrate, however, the small active silicon area reduces thepossibility to define an n-type floating region without degrading thedevice Rdson. Consequently, only the effect of the field plate ispossible and the doping concentration of the drift region which sustainsthe voltage has to be lowered, leading to an increase of Rdson.

SUMMARY

A p-channel LDMOS device with a controlled n-type buried layer (NBL) isdisclosed. Shallow trench isolation (STI) oxidation is defined,partially or totally covering the drift region length. The NBL layer,which can be defined with a p-well mask, connects to the n-welldiffusion, thus providing an evacuation path for electrons generated byimpact ionization. High immunity to the Kirk effect is achieved,resulting in a significantly improved safe-operating-area (SOA). Theaddition of the NBL deep inside the drift region supports a space-chargedepletion region which increases the RESURF effectiveness, thusimproving BV. An optimum NBL implanted dose can be set to ensure fullycompensated charge balance among n and p doping in the drift region(charge balance conditions). Since the drift depletion action isenhanced with the addition of the NBL layer, the p-well implanted dosecan be further increased to maintain a charge balance, which leads to anRdson reduction.

The p-channel LDMOS with additional NBL improves the BV versus Rdsontrade-off in p-channel LDMOS transistors and consumes less surface area,resulting in lower cost transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing the structure of a conventionalp-channel LDMOS device.

FIG. 2 is a cross sectional view showing the structure of a p-channelLDMOS device, including an NBL.

FIG. 3 is a flow diagram of a process for fabricating the p-channelLDMOS of FIG. 2.

FIG. 4 is a schematic diagram illustrating charge distribution along aSilicon/BOX/Silicon drift region in the proposed p-channel LDMOSstructure.

FIG. 5 is a graph of calculated constant doping concentration in thep-well and NBL layers as a function of the p-well (T_(pwell)) and NBL(T_(NBL)) thickness.

DETAILED DESCRIPTION

FIG. 1 is a cross sectional view showing the structure of a conventionalp-channel LDMOS 100. LDMOS 100, which is an SOI LDMOS, includessubstrate 122, buried oxide layer (BOX) 120, n-well region 102, p-wellregion 118, n-doped active region 106, p-doped active regions 104, 116,source 108, gate 110 and drain 114.

BOX 120 (e.g., SiO2) is formed in substrate 122 using, for example, aSeparation by IMplantation of OXygen process (SIMOX). Substrate 122 caninclude a semiconductor material such as silicon (Si) orsilicon-germanium (SiGe). In this example, substrate 122 is doped withn-type impurities.

P-channel LDMOS 100 is fabricated in n-type substrate 122. An n-dopedlayer is formed in substrate 122 as n-well region 102, and a p-dopedlayer is formed inside n-well region 102 forming drift region 118(hereinafter also referred to as p-well region 118). N-well region 102and drift region 118 can be formed in substrate 122 overlying BOX 120using implantation and photoresist masks. Some examples of n-typeimpurities are Phosphorous and/or Arsenic. Likewise, drift region 118can be implanted with a p-type impurity using a photoresist mask. Someexamples of p-type impurities are Boron and/or Indium. Drift region 118is laterally adjoining n-well region 102.

N-doped active region 106, which forms an n-well contact region, can beformed in n-well region 102 using implantation and a photoresist mask.N-well 102 is connected to source 108 terminal through n-doped activeregion 106 to avoid the activation of inherent parasitic bipolartransistor activation by short-circuiting n-well region 102.

P-doped active region 104, which forms a contact region for source 108,is formed in n-well region 102 using implantation and a photoresistmask. N-doped active region 106 and p-doped region 104 can be connectedtogether at the same potential. P-doped active region 116, which forms acontact region for drain 114, can be formed in p-well region 118 usingimplantation and a photoresist mask.

Source 108 at least partially overlies n-doped active region 106 andp-doped active region 104. Gate 110 at least partially overlies p-dopedactive region 104 and n-well region 102. Gate 110 can include dopedpolysilicon disposed in one or more layers of dielectric material (e.g.,silicon oxide) or other known conductive materials. Drain 114 at leastpartially overlies p-doped region 116. A shallow trench isolation (STI)oxidation 124 spaces drain 114 and gate 110 apart, so that a highdrain-to-gate voltage can be applied. Electrodes of conductive materialcan be disposed on source 108, drain 114 and gate 110.

Gate 110 can be used to induce a field-enhanced depletion region betweensource 108 and drain 114, and thereby creating a “channel.” Channelcurrent can be controlled by a vertical electric field induced by gate110 and a lateral field that exists between source 108 and drain 114.

P-channel LDMOS 100 has a low optimal drift doping concentration whichleads to a high Rdson, as compared to an n-channel LDMOS. To optimizethe BV and improve the Rdson/BV trade-off of the p-channel LDMOS 100structure, a new drift region design is needed that includes an NBL, asdescribed in reference to FIG. 2.

FIG. 2 is a cross sectional view showing the structure of a p-channelLDMOS device 200, including an NBL 202. P-channel LDMOS 200 has the samestructure as p-channel LDMOS 100 except for the inclusion of NBL 202.

In some implementations, NBL 202 can be included in substrate 122 usinga high-energy Phosphorus and Boron multi-implantation sequence. NBL 202can be underlying p-well region 104 and vertically adjoining BOX 120.STI 124 can be previously defined in substrate 122, partially or totallycovering the length of drift region 118. STI 124 can be defined by thefollowing process steps: stack deposition (oxide+protective nitride),lithography print, dry etch, trench fill with oxide, chemical-mechanicalpolishing of the oxide, removal of the protective nitride and adjustingthe oxide height.

NBL 202 can be defined with a p-well mask, so that it connects to n-welldiffusion, thus providing an evacuation path for electrons generated byimpact ionization. NBL 202 provides high immunity to the well-known Kirkeffect, thus significantly improving the safe-operating area (SOA). Theaddition of NBL 202 deep inside drift region 118 supports a space-chargedepletion region which increases the RESURF effectiveness, thusimproving BV. An optimal NBL 202 implanted dose can be set to ensurefull depletion before breakdown, thus achieving good reliabilityconditions with a compensated charge balance among n and p doping indrift region 118 (charge balance conditions). Since the drift depletionaction is enhanced with the addition of NBL 202, the p-well implanteddose can be further increased to maintain charge balance, which leads toRdson reduction.

FIG. 3 is a flow diagram of a process 300 for fabricating the p-channelLDMOS of FIG. 2. In some implementations, process 300 can begin bypreparing an SOI substrate including a BOX (302). The SOI substrate canbe prepared using SIMOX, for example. STI is formed in the substratepartially or totally overlying the drift region (304). An n-well regionis formed in the SOI substrate, so that the n-well region is verticallyadjoining the BOX (306). A p-well region is formed in the n-well regionto provide a drift region including an NBL disposed between the BOX andthe drift region (308). The NBL is laterally connected to the n-wellregion to provide an evacuation path for electrons generated by impactionization. An n-doped region is formed in the n-well region to form acontact region for a source (310). A p-doped region is formed in thedrift region to form a contact region for a drain (312). A gate isformed in dielectric material disposed on the top surface of the SOIsubstrate so that the gate region at least partially overlies the n-wellregion and the drift region, defining a channel (314). In someimplementations, electrodes formed from conductive material can bedisposed on the source, gate and drain, respectively, using ametallization process.

FIG. 4 is a schematic diagram illustrating charge distribution along theSilicon/BOX/Silicon region in the proposed p-channel LDMOS structure.For example, in thin film SOI substrates with a Silicon active thickness(T_(SOI)) of 1.6 um, NBL 202 thickness (T_(NBL)) can be made as small aspossible to not excessively reduce the drift current path which couldpenalize Rdson. As shown in FIG. 4, NBL 202 is depleted by the combinedaction of substrate field-effect action, and the p-well/NBL junction.Then, the optimal NBL implanted charge can be appropriately chosen tocompensate both depletion effects. Hence, the total depletion process inNBL 202 can be defined by

W _(NBL) =W _(N/P) +W _(ox/N),

where W_(N/P) is the depletion region in NBL 202 due to the p-well/NBLjunction diode represented by

$\begin{matrix}{{W_{N/P} = {\sqrt{\frac{2 \cdot ɛ_{si}}{{q \cdot N}\; B\; {L\left( {1 + \frac{N_{NBL}}{N_{pwell}}} \right)}}\left( {V_{D} - V_{bi}} \right)} \approx \sqrt{\frac{ɛ_{si} \cdot V_{D}}{q \cdot N_{NBL}}}}},} & \lbrack 2\rbrack\end{matrix}$

where N_(NBL) and N_(pwell) are the constant doping concentration of theNBL and p-well layers, respectively, and W_(ox/N) is the depletionregion in NBL 202 due to the SOI substrate field effect action, definedby

$\begin{matrix}{{W_{{ox}/N} = \frac{ɛ_{si} \cdot ɛ_{ox} \cdot V_{D}}{q \cdot N_{NBL} \cdot \left( {{ɛ_{si} \cdot T_{BOX}} + {ɛ_{ox} \cdot T_{NBL}}} \right)}},} & \lbrack 3\rbrack\end{matrix}$

where ∈_(si) is the dielectric constant of Silicon, ∈_(ox) is thedielectric constant of oxide, q is the electronic charge, V_(bi) is thebuilt-in potential, V_(D) is the threshold voltage and T_(BOX) is thethickness of the BOX.

In optimal RESURF conditions, the vertical full depletion of the NBLregion has to take place before the junction breakdown. The voltagecapability of a planar junction in a lateral diode is given by:

$\begin{matrix}{{V_{BR}^{Ld} = \frac{ɛ_{si} \cdot E_{c}^{2}}{2 \cdot q \cdot N_{NBL}}},} & \lbrack 4\rbrack\end{matrix}$

where ∈_(si) is the dielectric constant of Silicon and E_(c) is theSilicon critical electric field (≅3×10⁵ V/cm). Then, if the planarbreakdown expression [4] is introduced into expression [1], and thetheoretical bound in 1D-RESURF theory is considered by settingN_(NBL)=N_(pwell) in expression [2], then the W_(NBL) can be simplifiedto the following expression:

$\begin{matrix}{W_{NBL} = {\frac{2 \times 10^{12}}{N_{NBL}}{\left( {\sqrt{\frac{1}{2}} + {\frac{1 \times 10^{12}}{N_{NBL}} \cdot \frac{ɛ_{ox}}{\left( {{ɛ_{si} \cdot T_{BOX}} + {ɛ_{ox} \cdot T_{NBL}}} \right)}}} \right).}}} & \lbrack 5\rbrack\end{matrix}$

FIG. 5 is a graph of calculated constant doping concentration in thep-well and NBL layers as a function of p-well (T_(pwell)) and NBL(T_(NBL)) thickness. The theoretical drift resistances (R_(LDD)) can beextracted from

$\begin{matrix}{{R_{LDD} = {\frac{1}{q \cdot N_{pwell} \cdot \mu_{p}} \cdot \left( \frac{L_{LDD}}{T_{Active} \cdot W} \right)}},} & \lbrack 6\rbrack\end{matrix}$

where L_(LDD) is the drift region andT_(Active)=T_(SOI)−(T_(NBL)−W_(P/N)) in the proposed p-channel LDMOS,and the depletion region in the p-well layer, W_(P/N), expressed as:

$\begin{matrix}{W_{P/N} = {\sqrt{\frac{ɛ_{si} \cdot V_{bi}}{q \cdot N_{pwell}}}.}} & \lbrack 7\rbrack\end{matrix}$

While this document contains many specific implementation details, theseshould not be construed as limitations on the scope what may be claimed,but rather as descriptions of features that may be specific toparticular embodiments. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable sub combination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a sub combination or variation of a sub combination.

1. A p-channel silicon-on-insulator (SOI) lateral double diffusedmetal-oxide-semiconductor (LDMOS) structure, comprising: a substrate; aburied oxide layer formed in the substrate; an n-well region formed inthe substrate and vertically adjoining the buried oxide layer; a p-wellregion formed in the n-well region and partially laterally adjoining then-well region, the p-well region being a drift region; and an n-typeburied layer (NBL) underlying the drift region and vertically adjoiningthe buried oxide layer.
 2. The semiconductor structure of claim 1,further comprising: an n-doped active region and a first p-doped activeregion formed in the n-well region and a second p-doped active regionformed in the p-well region.
 3. The semiconductor structure of claim 2,further comprising: a source at least partially overlying the n-dopedactive region and the first p-doped active region, a gate at leastpartially overlying the n-well region and the p-well region and a drainat least partially overlying the second p-doped active region.
 4. Thesemiconductor structure of claim 1, further comprising: a shallow trenchisolation (STI) oxidation spacing the drain and gate apart.
 5. A methodof fabricating a p-channel silicon-on-insulator (SOI) lateral doublediffused metal-oxide-semiconductor (LDMOS) structure, comprising:forming an n-well region on an SOI substrate; forming a shallow trenchisolation (STI) at least partially in the drift region; forming a p-wellregion in the n-well region to provide a drift region, the p-well regionincluding an n-type buried layer disposed between the Box and the p-wellregion; forming an n-doped active region in the n-well region, then-doped active region forming a source; forming a first p-doped activeregion in the drift region, the first p-doped active region forming adrain; forming dielectric material at least partially over the n-wellregion and drift region; and forming a gate in the dielectric material,the gate partially overlying the n-well region and the STI oxidation.